A rigid endoscope usually has an optical system consisting of an objective, an ocular and between them a relay lens system consisting of several relay sets. Because the objective and each relay set is producing an image which is turned upside down, and because a standard endoscope should produce an upright image, usually an odd number of relay sets is used so that the image produced by the optical system is upright.
For good image quality, the optical system of an endoscope should be well corrected for the major lens aberrations. Most of the aberrations are corrected by the symmetrical design of the relay set. But it is especially difficult to correct for curvature of field aberrations.
U.S. Pat. No. 4,693,568 shows a low cost relay set of simple design having in each half set two lens units of positive power. The disadvantage of this design is that it cannot correct the curvature of field aberration.
From U.S. Pat. No. 4,676,606, a relay set of the generic type is known. In each half set it comprises a lens unit of negative power. This negative power element is responsible for correction of field of curvature. The aberration produced by the positive elements is corrected by the negative element. The advantage of this design is that the curvature of field correction provided by this relay set can be used to have the relay set well corrected for curvature of field aberration. The correction also can be made stronger so that this relay set is not only correcting its own curvature of field aberration but also the aberration of other optical elements of the optical system. It can be used together with relay sets of different types like that shown in U.S. Pat. No. 4,693,568 and can provide the overall correction of the optical system.
The disadvantage of the system of U.S. Pat. No. 4,676,606 is that the negative power element is of the well known Gauss type. This optical design is highly complex and extremely expensive and therefore not suited for mass produced endoscopes.